Noninductive cathode heaters



Jan. 13, 1959 J. TURNER 2,869,032 NONINDUCTIVE CATI-IODE HEATERS Filed April 29, 1957 V Y Z/ g /0- /3 Z z g Z 7 76/ v Z 7 5 5;

//\/l E/VTO/Q JOHN L. TURNER ATTORNEY NQNZNDUCTWE CATHODE HEATERS John L. Turner, Needham, Mass, assignor to Raytheon Manufacturing Company, Waitham, Mass, a corporatron of Delaware Application April 29, 1957, Serial No. 655,815 2 Claims. (Cl. 315--39.51)

This invention concerns a noninductive heater for a cathode and, more particularly, a heater for a magnetron cathode.

In magnetron oscillators, the frequency of the oscillatory output is a function of the intensity of the magnetic field within the interaction space between the central cathode and the surrounding anode. It is most convenient to supply the heater energy as an alternating current from a transformer with its primary connected to power lines. However, in a directly-heated cathode where the heating current flows through the cathode, a magnetic field is set up that varies from a field of a polarity to aid the main magnetic field to one of a polarity to oppose the main magnetic field. This varying magnetic field can cause some frequency modulation of the radio-frequency output of the magnetron. This frequency modulation may be of sufficient magnitude to be undesirable in certain applications. In an indirectlyheated cathode, the filament is frequently a bifilar Winding. that is, the filament is folded and then wound. The bifilar winding cancels any magnetic field set up by the coil filament. However, there is a potential difference between the two parts of the winding which is greatest at the point where the two ends of the winding leave the cathode. To prevent shorting-out the filament, the two parts of the winding must be insulated from each other. When the winding is subjected to shock vibration or thermal stress, this insulation may break down, shorting out the heater and causing failure of the magnetron.

This problem is avoided by the heater construction of the invention in which the heater is constructed of two coils wound in opposite directions with one nested within the other and connected in parallel. Because of the opposing directions of the windings and their almost identical geometry, the resulting alternating magnetic field is negligible. There is no appreciable difference in potential between adjacent portions of the two coils and so no need for any insulation which might be destroyed by shock vibration and thermal strains. Other and further objects, advantages andfeatures of the invention Will be better understood from the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a schematic view in section of the interaction space of a magnetron; and

Fig. 2 is an exploded View of the heater of the invention.

In Fig. 1, the reference numeral it) designates a conductive cathode sleeve suspended within openings 11 and 12 in pole pieces 13 and 14 forming part of the magnetic circuit of a magnetron of any of several well-known designs. The central portion of the cathode sleeve 10 has a coating 15 of electron-emissive material. The cathode id is coaxially located within the radially-extending vanes 16 of the anode of the magnetron. The cathode 10 is heated by a heater 17 comprising two filaments 18 and 2t) wound in opposite directions, with the outside diameter of one coil sufiiciently small to permit it to fit ates Patent 2,869,032 Patented Jan. 13, 1959 ice into the inside diameter of the other coil. The size of the wires of the two coils should preferably be selected so that, although the wire in the two coils is of different length, they will carry the same current. The upper ends of the two coils are connected together and to a connector 21 adapted to be brought out through the opening 11 in the pole piece 13 to a source of heater potential (not shown). The other ends of the coils 18 and 20 are connected together and to a conductive piece 21 connected to the cathode sleeve 10. It is also possible to connect these ends to a conductor, such as conductor 21, if it is desired to heat the cathode 10 without having the heating current pass through the cathode itself.

In operation, the heater current flows through both coils 18 and 20 in the same direction, but, because of the opposing directions of the windings of the two coils, the magnetic force produced by one coil is opposed by that produced by the other coil, and so the resuitant magnetic force due to these filament windings is reduced to a minimum, and, consequently, there is no variation in frequency of the magnetron output from this source. Also, as the coils are connected in parallel, the voltages at adjacent points on each coil are equal, and there is no need to insulate the coils from each other. Thus, the problem of insulation breaking down under the vibration, shock and thermal stress with the consequent shorting-out of the heating current is avoided. This is important where high filament voltages are used in order to operate the cathode at a high temperature. The construction of this invention is easier to make with consistent results than some existing constructions. The structure of this invention can be substituted for the standard suspended coil heater with a minimum of other design changes.

Thisinvention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention Within the art.

What is claimed is:

1. An electron discharge device of the magnetron type comprising a cathode structure, an anode structure incorporating a cavity resonator spaced from said cathode structure, and means adjacent said cathode and anode structures for establishing magnetic field in. a direction transverse to the discharge path therebetween, said cathode structure including a conductive sleeve provided with an electron emissive coating, and a heater comprising two oppositely wound conductive heating coils, a conductive input means, said heating coils coaxially mounted one within the other inside said cathode sleeve, a first pair of adjacent ends of said coils connected to said input means, the other pair of adjacent ends of said coils connected to said conductive sleeve, thereby connecting said coils in parallel between said sleeve and said input means.

2. A heater means for a hollow cylindrical cathode structure for use in a magnetron comprising a pair of oppositely Wound conductive heating coils, a conductive input means, said heating coils coaxially mounted inside said cathode structure one within the other, a first pair of adjacent ends of said coils connected to said input means, the other pair of adjacent ends of said coils connected to said cathode structure, thereby connecting said coils in parallel between said cathode structure and said input means.

References Cited in the file of this patent UNITED STATES PATENTS Lenz Nov. 10, 1936 

